In general, nitrogen oxide (NOx) included in exhaust gas causes acid rain, irritates eyes and respiratory organs, and kills plants. The NOx is regulated as main air pollutants, and a number of researches are being conducted to reduce emission of the NOx.
An exhaust gas recirculation (EGR) system is a system mounted in a vehicle in order to reduce deleterious exhaust gas. In general, the amount of NOx increases in a case in which a proportion of air to the gaseous mixture is large and thus combustion is smoothly carried out. Therefore, the exhaust gas recirculation system is a system that mixes a part (e.g., 5 to 20%) of exhaust gas discharged from the engine with the gaseous mixture again so as to reduce the amount of oxygen in the gaseous mixture and hinder the combustion, thereby inhibiting generation of NOx.
As the exhaust gas recirculation system, there are a hot exhaust gas recirculation system (hot EGR system) which reduces particulate materials (PM) and hydrocarbon (HO) as well as NOx by delaying ignition timing and reducing an air-fuel ratio, and a cooled exhaust gas recirculation system (cooled EGR system) which hinders combustion by cooling recirculating exhaust gas and decreasing a temperature in a combustion chamber.
An EGR ratio refers to a ratio of the amount of recirculation gas introduced into the engine through the exhaust gas recirculation system to the overall amount of air (fresh air+recirculation gas) introduced into the engine. When the EGR ratio increases, a concentration of oxygen supplied into the combustion chamber is decreased, such that a combustion temperature is decreased, and the amount of nitrogen oxide is decreased. However, when the amount of recirculating exhaust gas is excessively increased, an output of the engine and fuel economy deteriorate. In contrast, when the EGR ratio decreases, the concentration of oxygen supplied into the combustion chamber is increased, such that a combustion temperature is increased, and the amount of nitrogen oxide is increased. Therefore, the amount of recirculating exhaust gas needs to be appropriately controlled.
In addition, after the engine is turned off, the recirculation gas remains in an intake flow path through which the recirculation gas and the fresh air flow. Further, when the engine is turned on again, the residual gas is introduced into the combustion chamber of the engine, and in this case, there occurs a problem of abnormal combustion such as knocking or pre-ignition.
The above information disclosed in this Background section is only for enhancement of understanding the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.